The present invention relates to a pump arrangement and, more particularly, to a self-priming centrifugal pump with an external arrangement for controlling mixture formation effective during a priming step. The pump includes a pump volute commencing at a position above a pump impeller and surrounding the pump impeller, which volute is adapted to conduct a conveyed liquid into a venting chamber arranged above the pump impeller. An air-liquid mixture, conveyed during a priming step, is unmixed by the utilization of a return-flow duct connecting the venting chamber with the pump volute, which return-flow duct terminates into the pump volute at a position below the pump impeller.
Centrifugal pumps of the aforementioned type have been purposed which are capable, with the aid of a stored liquid remaining in the pump after a shutdown, of venting an empty suction or intake line within a certain time period during a restarting process of the pump.
A disadvantage of known pumps of the aforementioned type, utilized for cooling purposes, resides in the fact that the procedure necessary for venting an emptied suction line takes too long and a suction head attainable in dependence upon the number of revolutions of the pump lies far below a possible maximum theoretical value. This is due to an inadequate venting of the air-liquid mixture in the venting chamber, the ventingbeing impeded by a constant feeding of a new mixture from the volute into the venting chamber. Consequently, during start up liquid still greatly enriched with air passes through the return-flow duct into the pump volute, and thus the pump volute can accept, at that point, only small amounts of air from the suction or intake line.
A further disadvantage of the known centrifugal pump construction resides in the fact that, due to the high proportion of air, return-flow ducts having large cross sections are required to enable a transporting of sufficient liquid into the pump volute. The result being that, during operation of the pump, a large internal liquid circulation occurs within the pump with a corresponding reduction in a delivery output and degree of efficiency of the pump.